The mood in the corridors at the Interior Department yesterday was "a tad surreal," in the words of one official, as employes absorbed the news that Secretary William P. Clark had decided to leave a post in which he seemed perfectly at ease.

"People were pretty surprised," said department spokesman Rusty Brashear, who had scotched repeated rumors of Clark's departure with so much assurance in recent weeks that reporters had been inclined to believe him.

Environmental lobbyists, many of whom had opposed Clark's nomination to a job for which he had few professional credentials, grudgingly acknowledged that they were sorry to see him go.

"We're disappointed that he's leaving," said Environmental Policy Institute President Louise C. Dunlap, who testified against Clark at his confirmation hearings. "We were pleasantly surprised by the way he has handled himself."

Paul Pritchard, president of the National Parks and Conservation Association, said Clark had "left his mark, a good one."

Underlying the accolades, however, was a deeper concern that the administration will replace Clark with someone cast more in the philosophical mold of his predecessor, James G. Watt, at a time when the Interior Department is rethinking many of the policies that Watt laid down early in President Reagan's first term.

"A lot of what Watt did has since been repudiated by industry, by Congress or by the courts," said Dunlap. "Whoever succeeds Clark is not carrying out a maintenance function, but will have to make major policy judgments."

Despite Clark's statement that his job was "substantially complete," for example, the department is still in the throes of formulating policy on wilderness protection and mineral leasing, and a federal court recently rejected the strip-mining rules that were rewritten on Watt's watch.

The Office of Management and Budget is seeking a fiscal 1986 budget for the department that would wipe out some of Congress' most beloved programs, from parkland purchases to stocking U.S. waters with sport fish. On top of that, four of the department's top officials have resigned, and Clark -- who doesn't expect to leave until March -- says he will let his successor name their replacements.

The unfinished business at Interior stems in part from the White House's desire to keep environmental issues out of the limelight during the past election year.

As a result, Clark's tenure has been marked by case-by-case decisions that were generally popular with conservationists, but carried few long-term policy implications.

He scaled back certain oil-lease sales, for example, and also halted coal leases while the department re-examined the program. But he never backed off the ultimate goal of Watt's ambitious energy development program, or substituted more moderate goals of his own.

"All of the energy-leasing programs are in some kind of trouble," said John McComb, conservation director of the Sierra Club, noting that large parts of Watt's program "have been substantially stalled by congressional action or some court action."

The wilderness program is in a similar spot, according to McComb.

Of most immediate concern among conservationists and department employes, however, is the fiscal 1986 budget. The OMB is seeking cuts of 25 to 40 percent in some programs, and would eliminate virtually all funds for buying parkland and wildlife refuges.

Brashear said that Clark had already appealed OMB's decisions, and will be "actively involved" in arguing for a different arrangement of cuts.

But some were skeptical that the department will get much defense from a lame-duck secretary or key aides with "acting" titles.

"One does get the impression, with the vacancies and all those leaving, that there will be no vigorous advocates," said McComb.